A known pressure sensor shown in U.S. Pat. No. 4,875,135, assigned to the assignee of the present invention, the subject matter of which is included herein by this reference, comprises a capacitive pressure transducer having a thin ceramic diaphragm mounted in closely spaced, sealed, overlying relation to a ceramic base. Metal coatings are deposited on respective opposing surfaces of the diaphragm and base to serve as capacitor plates arranged in predetermined closely spaced relation to each other to form a capacitor. Transducer terminals connected to the capacitor plates are arranged at an opposite surface of the transducer base and a signal conditioning electronic circuit connected to the transducer terminals is mounted on the transducer. A connector body of electrical insulating material is fitted over the electronic circuit and is secured to a metal housing having a cavity in which the transducer is received. The metal housing is formed with a port for connection to a fluid pressure source to be monitored and a flexible O-ring is seated in the metal housing around the port with the transducer biased against the O-ring to form a fluid seal and to define a fluid receiving recess with the diaphragm exposed to fluid in the recess. In that arrangement, the diaphragm is movable in response to variation in pressure applied to the diaphragm to vary the capacitance of the capacitor in accordance with the changes in applied pressure and the electrical circuit provides an electrical output signal corresponding to the applied pressure.
Although sensors of the type described have been very successful, there are certain fluids which are corrosive relative to elastomers used for the O-ring seals. By way of example, fluids having ammonia and automotive fluids having certain additives are incompatible with typical elastomers.
It is conventional to place a sensing element in a chamber and to seal the chamber with a flexible diaphragm with the chamber being filled with relatively incompressible oil to serve as a pressure transfer fluid. This approach could be used in sensing a condition of a fluid to be sensed, such as pressure, which fluid is corrosive to the sensing element. However, this represents a relatively expensive solution in manufacturing a suitable package requiring a number of processing steps including evacuation of a chamber and back filling with oil and providing a seal compatible with the pressure of the sensed fluids.
Another pressure sensor shown in U.S. Pat. No. 6,273,927, assigned to the assignee of the present invention, the subject matter of which is included herein by this reference, comprises a capacitive pressure transducer having a first surface to be exposed to an applied pressure to provide a capacitance value which varies with the fluid pressure applied, a housing having a transducer receiving cavity defined by a bottom wall and a sidewall extending upwardly from the bottom wall, the bottom wall formed with a fluid pressure receiving recess, a fluid pressure port formed in the housing in communication with the recess, a separate thin, flexible metal diaphragm secured to the bottom wall circumscribing the recess in fluid sealing relation therewith, the transducer received in the transducer receiving cavity with the first surface overlying the bottom wall and a thin layer of synthetic material disposed intermediate to the first surface of the capacitive pressure transducer and the metal diaphragm and in direct contact with both of them as a pressure transfer element.
The sensor is a hermetically sealed device in which the sensed media is sealed from the sensing element. It has been proven to be a successful sensor design, but in certain instances, especially in below atmospheric pressure applications, it can have accuracy issues and assembly of the device can require precise control.